Heat generator for a motor vehicle using induction heating

ABSTRACT

A heat generator for reducing emissions includes a permanently magnetized, disc-shaped rotor; a stator axially separated from the rotor and in which the rotor induces electric currents as it rotates to generate heat in the stator; and an adjoining cooling duct through which a cooling liquid flows to dissipate heat generated in the stator. The rotor and stator are axially movable in relation to one another to adjust the heat generated in the stator. Heat output is controlled by a member acting axially on the rotor in the direction of the stator with a variable force against the action of a repelling force, and includes a soft magnetic material forming part of the stator. The permanently magnetized rotor and the stator are magnetically attracted to one another, and it is possible to strengthen/weaken their to achieve a predetermined output/speed profile.

This application is the national phase under 35 U.S.C. §371 of PCTInternational Application No. PCT/SE00/01262 which has an Internationalfiling date of Jun. 16, 2000, which designated the United States ofAmerica and was published in English.

The present invention relates generally to a heat generator for motorvehicles and primarily to one for reducing emissions from the internalcombustion engine of a motor vehicle.

U.S. Pat. No. 4,484,049 discloses a water-cooled heat generator for thepassenger compartment of a motor vehicle. The heat generator comprises ashaft driven by the vehicle engine, which shaft is common to the rotorin an electric generator and a rotor in the heat generator itself.Alternating current drawn from the stator winding of the electricgenerator is rectified and fed as magnetising current to the rotor inthe heat generator. The latter also has a laminated stator with armaturebars connected between two short-circuit rings, the bars like theshort-circuit rings being hollow. The armature bars, in which the rotorof the heat generator generates induction currents as it rotates, are,like the short-circuit rings, cooled by water that circulates throughthem. The water thus heated is in turn used for heating the vehiclepassenger compartment.

The said heat generator is bulky, complicated and also has poorefficiency.

U.S. Pat. No. 5,573,184 discloses a heat generator based on frictionheating. This heat generator, too, is bulky owing to a highvolume/output ratio, a relatively high-viscosity operating liquidmoreover being required.

PCT/SE99/00283 describes a heat generator with a permanently magnetised,disc-shaped rotor, a stator, which is axially separated from the rotorand in which the rotor induces electric currents as it rotates, whichgenerate heat in the stator, and adjoining the stator a cooling duct fora flowing liquid in order to dissipate the heat generated in the stator.

This heat generator is compact and provides very efficient heating ofthe cooling liquid, which is intended for use in heating the coolant inan internal combustion engine in an initial phase after starting theinternal combustion engine.

Because of the highly efficient conversion from mechanical work, whichdrives the heat generator, to generated heat in the cooling liquid, itmay be necessary in many applications to control the generated heatoutput according to a varying demand. This might obviously be done bycorresponding variation of the mechanical power delivery, which drivesthe heat generator, by adjusting the speed of rotation of the rotor ofthe heat generator. Such a speed variation is not possible in allapplications, however, since other requirements may be crucial for themagnitude of the mechanical power output, for example the power neededto propel a vehicle. In such a case the desired heat output cannevertheless be produced by connecting the heat generator intermittentlyin such a way that the mean value of the generated heat outputcorresponds to the desired heat output. This intermittent connection,however, presupposes a connecting arrangement, which requires space andmay mean that the heat generator becomes complicated and/or undesirablyexpensive.

An object of the present invention is to provide a heat generator formotor vehicles, which is compact but still permits easy adjustment ofthe heat generated.

A heat generator according to the invention has a permanentlymagnetised, disc-shaped rotor; a stator, which is axially separated fromthe rotor and in which the rotor induces electric currents as itrotates, which generate heat in the stator; and adjoining the stator acooling duct for a flowing cooling liquid in order to dissipate the heatgenerated in the stator. The rotor and the stator are furthermoreaxially moveable in relation to one another, in order to adjust thedistance between them and thereby to adjust the heat generated in thestator or the heat output. This makes use of the fact that an adjustmentof the distance between the rotor and the stator affects the magnitudeof the currents. that the rotor induces in the stator and hence themagnitude of the heat generated by the said currents. Finally, means arearranged for determining the heat output generated in the stator byacting axially on the rotor in the direction of the stator with avariable force against the action of a repelling force, generated by thecurrents induced in the stator. The rotor can thereby be set to an axialposition in relation to the stator depending on the heat output to begenerated in the stator.

The stator preferably comprises two metallic layers, which define anarrow, substantially radial gap, which constitutes a part of thecooling duct designed for radial flow. In this way the distance overwhich the liquid in the cooling duct is heated is rendered relativelyshort, which is a pre-requisite for rapid heating utilising a highoutput.

In addition, the cooling duct suitably comprises two annular spaces,which adjoin the radial gap on that side thereof remote from the rotorand are designed for a circumferential flow of the cooling liquid. Thisdesign means that the cooling liquid follows intersecting paths on bothsides of one of the two metallic layers, represented by the layerfurthest from the rotor. Because of these intersecting flows of coolingliquid, such phenomena as evaporation and film boiling, which mightotherwise occur as a result of the rapid heating and lead to cavitationand overheating, can be prevented.

The means for acting axially on the rotor in the direction of thestator, that is to say for adjusting the distance between the rotor andthe stator depending, for example, on the desired heating of the flowingliquid, may be designed in many different ways, but they suitablycomprise a soft magnetic material, which constitutes part of the stator,so that the magnetic field of the rotor is closed and a magneticattraction force acts between the permanently magnetised rotor and thestator.

Of the two metallic layers, one nearest the rotor may comprise anelectrically conductive, preferably non-magnetic material, and onefurthest away from the rotor may comprise the soft magnetic material.

The force generated in the stator, which has a repelling action on therotor, may be dimensioned so that at a predetermined speed of rotationit moves the rotor away from the stator against the action of themagnetic attraction force between the permanently magnetised rotor andthe stator. This is achieved, for example, through the choice of statormagnetic material and through the choice of its volume and distance fromthe rotor.

The means for acting axially on the rotor may comprise spring means,pneumatic or hydraulic piston and cylinder units and/or electricallyoperated units. The said means may strengthen and/or weaken the magneticattraction force or the repelling force in order to permit theachievement of a predetermined output/speed profile.

The means for acting axially on the rotor may furthermore comprise anoverride means for positively shifting the rotor to a desired outputposition or locking the rotor in a lower output position, for examplewhen the entire output of the car engine is required for acceleration.

The means acting axially on the rotor may furthermore be controlled byan override signal from control electronics, which control the internalcombustion engine, for limiting the heat output generated in the statorby positively shifting the rotor in relation to the stator, andachieving a position with limited output. Such an override signal mightbe generated by the control electronics in connection with acceleration,for example from stationary or in excess of a predetermined limit, therotor being shifted to and locked in a lower output position.

By giving the means for applying force to the rotor in the direction ofthe stator a suitable force/distance profile, the heat output generatedin the stator can thus take on a predetermined output/speed profile.

The currents that the rotor induces in the stator increase normally withincreasing speed. In order to take account of this increased current,the distance of the rotor from the stator can also be suitably increasedas the speed of the rotor increases. This increase may be used, forexample, to achieve the desired heat output/speed profile for anadjusted rotor speed. In particular, the increase in the distancebetween the rotor and the stator may be initiated only in excess of apredetermined speed, in order thereby to limit the maximum output.

In another embodiment, the means for applying force to the rotor in thedirection of the stator comprise a pneumatic piston, the force of whichmay be produced by negative pressure or excess pressure. If produced bynegative pressure, the pneumatic piston may advantageously be combinedwith spring means, which likewise exert force on the rotor, and morespecifically a diminishing force as the distance between the rotor andthe stator increases. It is especially advantageous if the forcecharacteristic of the said spring means is designed so that, at aconstant rotor speed, an increasing negative pressure in the pneumaticpiston moves the rotor closer to the stator.

As a safeguard against excessive heating of the cooling liquid,adjusting means may be provided alongside the cooling duct, which meansare sensitive to the temperature of the cooling liquid and have theability, when a predetermined temperature is reached, to act largelyinstantaneously on the rotor and to increase its distance from thestator to a value at which the heat generated in the stator isnegligible. In this way, heating of the liquid flowing in the coolingduct in excess of a predetermined temperature value can be prevented,for example, where water is used as cooling medium, heating in excess of100° C. can be prevented, thereby preventing evaporation at normalpressure.

The design of the cooling duct is critical with regard to the highoutput (with an order of magnitude of tens of kilowatts) that a heatgenerator according to the invention can develop. According to theinvention, short flow paths are produced in the most active heating zoneclosest to the rotor by designing the stator with a first disc extendingradially along the disc-shaped rotor and with a second disc situatedalong the said first disc and on the opposite side to the rotor, inorder to form a radial gap between them. The cooling liquid can be madeto flow radially through the said gap in that it is connected toopenings around its outer and its inner circumference.

In a further embodiment of the heat generator, the rotor comprises twoaxially separated rotor discs and the stator two stator discs, which arearranged between the two rotor discs and adjacent to a respective one ofthese. In this case distance adjustments are produced in that the rotordiscs are axially moveable away from one another and from the respectivestator disc, which is fixed in relation to the axial movement.

On each rotor disc the permanently magnetised, disc-shaped rotor mayhave a plurality of permanent magnets, and may for the rest consist of asoft magnetic material, and each stator disc may consist of anelectrically conductive material. As the rotor rotates, the magneticfield generated by the permanent magnets gives rise to currents in thestator discs, which currents in addition to generating heat in thestator discs in turn also give rise to magnetic fields that endeavour tocounteract the magnetic field generated by the rotor magnets. Themagnetic fields counteracting one another give rise to repelling forces,which act axially between the stator and the rotor, especially each pairof stator discs and each pair of rotor discs. These repelling forcingincrease as the rotor speed increases and decrease as the distancebetween rotor and stator increases.

In the case with the two stator discs and two rotor discs, the coolingduct comprises two radial gaps, which are formed between each statordisc and a disc of magnetic material fixed alongside this. The saidfixed disc of magnetic material is annular and closes the flux path forthe magnetic field from the rotor through the stator. An attractionforce thereby also occurs between. each rotor disc and the associatedfixed disc, which force endeavours to move the rotor discs towards therespective stator disc. This attraction force is largely dependent onlyon the distance.

Some examples of embodiments of a heat generator according to theinvention will be described in more detail below with reference to theaccompanying drawings.

FIG. 1 is a side view of a first embodiment of a heat generatoraccording to the present invention.

FIGS. 2 and 3 are axial, partial cross-sectional views taken along linesII—II and III—III respectively in FIG. 1.

FIGS. 4 and 5 are radial cross-sectional views taken along lines IV—IVand V—V respectively in FIG. 2.

FIGS. 6 and 7 are axial cross-sectional views of a second and a thirdembodiment of a heat generator according to the invention.

FIG. 8 is an axial cross-sectional view of a fourth embodiment of a heatgenerator according to the invention.

FIG. 9 is a perspective view of a stator forming part of the embodimentaccording to FIG. 8.

FIG. 10 is a perspective view and shows a heat generator according tothe invention mounted on a crankshaft of a partially shown internalcombustion engine for a motor vehicle.

FIG. 11 is an exploded view of a fifth embodiment.

FIG. 12 is a diagram of the output developed as a function of the speedand distance in an embodiment of a heat generator according to theinvention.

FIG. 13 is a diagram of repelling force as a function of the speed anddistance in an embodiment of a heat generator according to theinvention.

The embodiment of a heat generator shown in FIGS. 1-5 of the drawingshas a casing 1, which comprises two dish-shaped shields 2 and 3 and anintermediate peripheral ring 4, which are held together by means of sixscrew joints 5. A shaft 6 is supported centrally in the casing 1 bymeans of a bearing 7, 8 in each shield 2 and 3 respectively. Outside thecasing 1, a belt pulley is fixed to the shaft 6, which inside thehousing 1 supports a rotor 9.

The rotor 9 has a central hub 10 with six radially aligned spokes 11, atthe outer ends of which two plane, annular rotor discs 12 and 13 aresupported at an axial distance from one another. The rotor discs 12, 13are guided for axial movement outwards and away from the position shownin FIGS. 2 and 3 by means of six guide pins 14, which are alternatelyfixed to the rotor disc 12 and the rotor disc 13. Sixtemperature-sensitive cylindrical means 15 are arranged axially betweenthe rotor discs 12 and 13, which each consist of an outer ring and aninner ring fixed thereto, as can be seen from FIGS. 2 and 3. Thecylindrical means 15 are of known type and increase their length by apredetermined value almost instantaneously when the ambient temperatureexceeds a predetermined value.

The rotor discs 12 and 13 are pressed towards the position shown inFIGS. 2 and 3, which is defined by their abutment against the outer endsof the spokes 11, by six bimetal springs 16 for each rotor disc. Thesprings 16 are each arranged between one of the rotor discs 12, 13 andone of two backing or counter discs 17, 18, situated axially outsidethese and fixed to the shaft 6.

On the outside a plurality of permanent magnets 19 is fitted on opposingsides of the rotor discs 12, 13, with axially aligned poles and withpolarity alternating from magnet to magnet in the circumferentialdirection of the rotor discs.

A stator 20 and a cooling duct 21 are situated between the rotor discs12, 13 and fixedly coupled to the casing 1. The stator 20 morespecifically has two stator discs 22, 23 in the form of plane rings ofelectrically conductive, preferably non-magnetic material, for examplecopper, which at their peripherally outer edge are fixed into the casing1, for example clamped between each of the shields 2, 3 and theperipheral ring 4, and at their peripherally inner edge are coupled to acasing 24, so that the stator discs 22, 23, the peripheral ring 4 andthe casing 24. form an annular space between permanent magnets 19 of therotor discs 12, 13, the said space constituting the cooling duct 21. Theannular space is divided up into an outer duct 25 and an inner duct 26by two plane rings 27, 28, which are fitted to the casing 24 and mayalso be said to form an integral part of the stator, and a substantiallysleeve-shaped ring 29, which is connected between the plane rings 27 and28 and extends around them and to a first opening 30 in the peripheralring 4, which is thereby connected to the inner duct 26. A secondopening 31 in the peripheral ring 4 is connected to the outer duct 25.

As will be seen from FIGS. 2 and 3, the plane rings 27, 28, whichpreferably consist of soft magnetic material, are arranged at a shortaxial distance from the respective stator discs 22, 23, so that tworadial gaps 32, 33 are formed between them, which connect the outer duct25 and the inner duct 26.

By means of the construction described above, a cooling medium, forexample water, can be pumped in through the opening 30, in thecircumferential direction into the inner duct 26, radially outwards viathe gaps 32, 33, in the circumferential direction through the outer duct25 to the second opening 31 and out through the same. The oppositedirection of flow, that is from the opening 31 to the opening 30, isnaturally also possible. With regard to this flow process, thecross-sectional area of the duct 26, that is to say its extent in theradial direction, may suitably diminish away from the opening 30, seenin the direction of flow, and the cross-sectional area of the duct 25may accordingly increase to a corresponding degree towards the opening31, seen in the direction of flow. The rate of flow in the longitudinaldirection of each of the ducts 25, 26 can thus be made largely constant.

With the rotor 9 at a standstill, when the rotor discs 12, 13 aresituated closest to one another, there is clearance between thepermanent magnets 19 of the rotor 9 and each adjacent stator disc 22,23. The rotor discs 12, 13 are then pressed into their position closestto each stator disc 22, 23 respectively, both by the compression springs16 and by the attraction force between the permanent magnets 19 and thefixed discs constituted by the plane rings 27, 28.

When the rotor 9 is made to rotate, the magnetic fields generated by thepermanent magnets 19 will generate electric currents in the stator discs22, 23, thereby powerfully heating the latter. The heat generated in thestator discs 22, 23 is dissipated by the cooling liquid flowing throughthe gaps 32, 33.

The electric currents in the stator discs 22, 23 will also give rise tomagnetic fields that generate a repelling force in relation to themagnetic fields of the permanent magnets 19, as a result of which therotor. discs 12, 13 are forced to move axially away from the respectivestator discs 22, 23 against the action of the compression springs 16 andthe attraction force between the permanent magnets 19 of the rotor discs12, 13 and the rings 27, 28.

By adjusting the spring characteristic of the compression springs 16 inrelation to the characteristics for the repelling or attracting magneticfields, a desired output/distance profile can be achieved.

The second embodiment shown in FIG. 6 differs from the first embodimentdescribed above in that, as abutment means for the compression springs16, two plane rings 34, 35 are used, which are fixedly connected to oneanother by means of pins 36, which extend through holes in the innerparts of the two rotor discs 12, 13. This provides a self-supporting andself-balancing abutment means for the compression springs 16.

In the third embodiment illustrated in FIG. 7, the helical springs 16are replaced by two star-shaped leaf springs 37, 38, which at theirradially inner part are fixed to the sides of a respective one of therotor discs 12, 13 remote from the stator 20. The leaf springs 37, 38may advantageously consist of bimetal and in addition be pretensioned soas to exert a compressive force against the rotor in the positionthereof shown in the figure. By making the leaf springs 37, 38 ofbimetal, the pressure that they exert can be made to gradually diminishas the temperature increases, the distance of the rotor from the statorincreasing and thereby reducing the heat developed in the stator, whichconsequently permits automatic control of the temperature of the liquidin. the cooling duct. Bimetal springs can also be made to switchinstantaneously from one stable position to another stable position at apredetermined temperature. By utilising this method of operation, theleaf springs 37, 38 can be made to function as protection againstexcessively high liquid temperatures in the cooling duct.

The embodiment of a heat generator according to the invention shown inFIGS. 8-9 differs from the preceding embodiments in that the rotor 9 hasonly one rotor disc 39. In this embodiment, a pneumatic piston 40, whichis axially moveable together with the rotor 9 but does not rotate withthe latter due to an intermediate ball bearing 41, is arranged in placeof the second rotor disc in the other embodiments. The inner space 42 ofthe piston 40 is connected to a vacuum source (not shown), which cancontrol the negative pressure in the piston 40.

A set 43 of springs, which may have varying hardness, is furthermorearranged inside the piston 40 and endeavours to move the piston 40 tothe left in FIG. 9, that is to move the rotor 9 away from the stator 20.The spring characteristic of the spring set 43 is designed so that thedifference between the forces acting on the rotor 9, which endeavour tomove the rotor 9 in the direction away from the stator 20, and theattraction force acting between the rotor 9 and the stator 20 due to thepermanent magnets 19 increases monotonously from the position of therotor 9 furthest away from the stator 20 to its position closest to thestator 20. As a result of this design, the position of the rotor 9 canbe easily and unambiguously controlled by means of the magnitude of thenegative pressure in the pneumatic piston 40, which negative pressurethus acts on the piston 40 and hence the rotor 9 in the directiontowards or away from the stator 20.

FIG. 10 illustrates how compact a heat generator 44 according to theinvention is in relation to a conventional internal combustion engine45. The heat generator 44, the stator 20 of which is fixed to theinternal combustion engine 45, may more specifically be coupled directlyto the engine shaft by way of the usual pulley 46 for the transmissionbelt 47, which couples the engine shaft to control servo and generator,for example. The inlet and outlet, that is the openings 30, 31, of theheat generator are connected by way of hoses 48, 49 to the coolingsystem of the internal combustion engine 45.

The embodiment of a heat generator described above may obviously bemodified in many respects, for example other means, such as electricallypowered means, can also be used for applying force to the rotor in thedirection of the stator. Such an embodiment is shown in the explodedview in FIG. 11. This embodiment of the heat generator is similar tothat in FIGS. 8 and 9 but instead of the pneumatic piston 40 and thespring set 43 has two meshing cog wheels 50, 51 with bevelled cams 52and cam grooves 53 respectively. The cog wheel 50 is supported so thatit is displaceable only axially and acts on the axial position of therotor 9, whilst the cog wheel 51 is supported so that it can onlyrotate, more specifically by means of an arm 54 which is fixedlyconnected to the cog wheel 51 and the rotational position of which canin turn be controlled, for example, by means of an electric motor 55according to an override signal from the control electronics, whichcontrol the internal combustion engine, for limiting the heat outputgenerated in the stator by positively turning the cog wheel 51 andthereby axially moving the cog wheel 50 and the rotor in relation to thestator, so that, for example, a position with a limited output can beachieved.

The cog wheel 50, 51 may naturally also form an override means forpositive movement of the rotor to a desired output position.

As a further alternative the cog wheel 51 may be made entirelystationary, that is to say axially immovable and also non-rotatable, andthe arm 54 may be non-rotationally but axially movably connected to thecog wheel 50. The cog wheel, as in the embodiment in FIG. 11, is in turnaxially moveable together with the rotor 9 and also rotatably supportedin relation thereto. This alternative is in fact one of the preferredalternatives.

FIG. 12 shows a diagram, which schematically illustrates the poweroutput W (kW) in a heat generator according to the invention as afunction of the speed of rotation r (rpm) at various distances d (d1-d4,for example 0.4/0.8/1.3/1.9 mm) between stator and rotor. It will beseen that the power output W increases with increasing speed of rotationat a constant distance (e.g. d1) between rotor and stator, and that therate of increase of the power output W is greater the smaller thedistance d between stator and rotor. Conversely, the power output Wdiminishes with increasing distance d at a constant speed of rotation r.A desired output/speed profile can thus be achieved by varying thedistance d according to the speed of rotation r.

Since the magnetic field strength varies substantially as the reciprocalof the square of the distance between the rotor and the stator, a veryappreciable variation in output can be obtained by a relatively smallvariation of the axial distance between the rotor and the stator. Thisin turn means that the heat generator according to the invention can bemade very compact.

FIG. 13 shows a diagram, which schematically illustrates the repellingforce F (N) in a heat generator according to the invention as a functionof the speed of rotation r (rpm) at various distances d (for exampled1-d4 according to the above) between stator and rotor. As describedabove, this repelling force is the result of the interaction between themagnetic fields of the permanent magnets and the magnetic field that isgenerated by the currents induced in the stator. It will be seen thatthe repelling force F increases with increasing speed of rotation at aconstant distance (e.g. d1) between rotor and stator and that the rateof increase of the repelling force F is greater the smaller the distanced between stator and rotor. Conversely the repelling force diminisheswith increasing distance d at a constant speed of rotation r.

In order to vary the distance d according to the speed of rotation rwith the aim of achieving a desired output/speed profile, the repellingforce is according to the invention suitably balanced by acting axiallyon the rotor in the direction of the stator with a variable force, whichis thus directed counter to the repelling force F.

In FIG. 12, a line is drawn for a constantly power output of. 12.5 kW(developed irrespective of the speed of rotation) and the intersectionsof the line with the power output curves for the various distances d1-d4are indicated by circles. The speed/distance intersections indicated bythe circles are also inserted in FIG. 13.

It will be seen that the repelling force diminishes with increasingspeed of rotation, when the power output is to be kept constant, whichmeans that the force that is required in order to balance the repellingforce will in this case also diminish correspondingly with increasingspeed of rotation.

It will be appreciated that a number of further modifications of theembodiments of a heat generator described above, for example variouscombinations of various means for applying force to the rotor in thedirection of the stator, are possible within the scope of the invention,as specified in the appended claims.

What is claimed is:
 1. A heat generator for reducing emissions frommotor vehicles, characterised by a permanently magnetised, disc-shapedrotor (9, 12, 13), a stator (20, 22, 23), which is axially separatedfrom the rotor and in which the rotor induces electric currents as itrotates, which generate heat in the stator, a cooling duct (21)adjoining the stator for a flowing cooling liquid in order to dissipatethe heat generated in the stator, the rotor (9, 12, 13) and the stator(20, 22, 23) being axially moveable in relation to one another in orderto adjust the heat generated in the stator, and means (15, 16, 19, 27,28, 37, 38, 40) for determining the heat output generated in the statorby acting axially on the rotor in the direction of the stator with avariable force against the action of a repelling force, which isgenerated by the currents induced in the stator, said means (15, 16, 19,27, 28, 37, 38, 40) comprising a soft magnetic material (27, 28), whichconstitutes part of the stator, so that a magnetic attraction force actsbetween the permanently magnetised rotor (9) and the stator (29), andmeans (15, 16, 19, 27, 28, 37, 38, 40) for strengthening and/orweakening the magnetic attraction force for achieving a predeterminedoutput/speed profile.
 2. A heat generator according to claim 1,characterised in that the stator (20) comprises two metallic layers (22,27), which define a narrow radial gap (32), which constitutes a part ofthe cooling duct (21) and is designed for a radial flow of the coolingliquid.
 3. A heat generator according to claim 2, characterised in thatthe cooling duct (21) additionally comprises two annular spaces (25, 26)adjacent to the radial gap, which are designed for a circumferentialflow of the cooling liquid.
 4. A heat generator according to claim 3,characterized in that the repelling force generated in the stator (20)is dimensioned, by means of the magnetic material of the stator and itsvolume and distance from the rotor (9), at a predetermined speed ofrotation, to move the rotor (9) away from the stator (20) against theaction of the magnetic attraction force between them.
 5. A heatgenerator according to claim 2, characterized in that the two metalliclayers (22,27) one layer (22) nearest the rotor (9) comprises anelectrically conductive, preferably non-magnetic material, and one layer(27) furthest away from the rotor (9) comprises a soft magneticmaterial.
 6. A heat generator according to claim 1, characterized inthat the means (15,16,19,27,28,37,38,40) acting axially on the rotor (9)comprise an override means (55) for positive movement of the rotor (9)to a desired output position.
 7. A heat generator according to claim 1,characterized by adjusting means (15) sensitive to the temperature inthe cooling duct (21) and having the capacity, on reaching apredetermined temperature, to increase the distance between the rotor(9,12,13) and the stator (20,22,23) largely instantaneously to a valueat which the heat generated in the stator is negligible.
 8. A heatgenerator according to claim 1, characterized in that the means(15,16,19,27,28,37,38,40) acting axially on the rotor (9) are controlledby an override signal from control electronics, which control theinternal combustion engine (45), in order to limit the heat outputgenerated in the stator (20) by positively moving the rotor (9) inrelation to the stator and achieve a position with limited output.
 9. Aheat generator according to claim 1, characterized in that the stator(20) comprises a first disc (22) extending radially along thedisc-shaped rotor (9), and a second disc (27) situated radially alongthe first stator disc and on the opposite side to the rotor in order toform a radial gap (32), which has trimming openings around its outer andits inner circumference in order to permit an evenly distributed flow ofthe flowing liquid through the gap (32) and dissipation of the heatgenerated in the stator (20).
 10. A heat generator according to claim 9,characterised by an outer annular space (25), which connects to theopenings at the outer circumference of the gap (32), and an innerannular space (26), which connects to the openings at the innercircumference of the gap (32), which annular spaces constitutecollecting lines for the liquid flowing to and from the gap (32)respectively.
 11. A heat generator according to claim 9, characterizedin that the openings are formed in the second disc (27).
 12. A heatgenerator according to claim 1, characterized in that the rotor(9,12,13) comprises two axially separated rotor discs (12,13) and thatthe stator (20,22,23) comprises two stator discs (22,23) arrangedbetween the two rotor discs and adjacent to a respective one of these.13. A heat generator according to claim 12, characterised in that therotor discs (12, 13) are axially moveable away from one another and fromthe respective stator discs (22, 23).
 14. A heat generator according toclaim 13, characterised in that each rotor disc (12, 13) has a pluralityof permanent magnets (19), that each stator disc (22, 23) comprises anelectrically conductive, preferably non-magnetic material, and that twofurther discs (27, 28) of magnetic material are arranged adjacent to arespective on of the stator discs (22, 23) and on the opposite side tothe respective rotor discs (12, 13).
 15. A heat generator according toclaim 1, characterized in that the cooling duct (21) is arranged to beconnected to the cooling system of an internal combustion engine (45),which is coupled so as to drive the rotor (9) and the emission values ofwhich are intended to be improved by this arrangement.
 16. A heatgenerator according to claim 1, characterized in that it constitutes anintegral part of the system of temperature and emission control of aninternal combustion engine, both mechanically, thermally and in terms ofcontrol engineering.
 17. A heat generator for reducing emissions frommotor vehicles, characterised by a permanently magnetised, disc-shapedrotor (9, 12, 13), a stator (20, 22, 23), which is axially separatedfrom the rotor and in which the rotor induces electric currents as itrotates, which generate heat in the stator, the rotor and the statorbeing axially moveable in relation to one another in order to adjust theheat generated in the stator, and a cooling duct (21) adjoining thestator and intended for a flowing cooling liquid for dissipation of theheat generated in the stator, which cooling duct comprises a narrowradial gap (32) adjoining the stator and two annular spaces (25, 26),each connected to a radial side of the radial gap, which spaces aredesigned for mutually opposing cooling liquid flows in thecircumferential direction of the cooling liquid, which thus flows fromone annular space (26) to the gap (32) and from this to the otherannular space (25).